GSA Special Paper

Cracking Grand Canyon's Rock Code

Boulder, CO, USA – In a new book published by The Geological Society of America, editors J. Michael Timmons of the New Mexico Bureau of Geology and Mineral Resources and Karl E. Karlstrom of the University of New Mexico, along with several individual chapter authors, describe the rock record of Grand Canyon and illustrate its complexity. For them, Grand Canyon, one of the premier geologic landscapes in the world, carries within its structure intriguing mysteries of time and space.

Compared to the vast history of Earth's evolution, the U.S. Grand Canyon, carved by the Colorado River and its tributaries, is believed to be relatively young. Yet, according to Timmons and Karlstrom, the canyon's rock record provides a unique sense of the "vastness of deep time."

Timmons, Karlstrom, and introductory chapter co-author Laura J. Crossey of the University of New Mexico write that while Grand Canyon is a "superb geologic laboratory for understanding geologic history," there is "more time that is not recorded by the rocks in Grand Canyon than that is recorded."

Missing episodes reflect erosion, and to comprehend the nature and extent of that erosion is to begin to put the pieces together to tell the complete story of the canyon. As Karlstrom and Timmons write, "The geologic record has been described as a 'fishnet,' where the twine of the net is the rock record and the holes are the time missing." The missing time, the holes, are called "unconformities."

The unconformities, including what geologists, beginning with John Wesley Powell in 1875, call Grand Canyon's Great Unconformity, were created by past geologic events, such as "vanished mountain ranges, cryptic uplift events, and unsung earthquakes."

In the volume’s introduction, Timmons, Karlstrom, and Crossey describe the record that can be seen in the canyon walls—three distinct series: (1) 270- to 525-million-year-old horizontal sedimentary rock layers that make up the upper strata throughout the canyon and represent multiple episodes of marine incursion and retreat from the continental interior; (2) 750,000-year-old to 1.2-million-year-old tilted rock layers, well preserved in fault blocks of eastern Grand Canyon, called the Grand Canyon Supergroup, revealing discrete episodes of sedimentation and tectonism; and (3) the oldest rocks, in the depths of the canyon—the igneous and metamorphic rocks that record the formation and modification of the region's continental crust, 1,660 to 1,840 million years ago (thus, the book’s subtitle: "Two Billion Years of Earth's History").

Nine chapters and an afterword continue to detail the visible as well as the no longer visible. Illustrated with color figures and photographs and accompanied by the most detailed geologic map of eastern Grand Canyon to date, this GSA Special Paper provides an easy-to-understand summary of the geologic stories encoded by rocks and landscapes of Grand Canyon.